.303 tracer was in use by the British since 1914

No it was not.
"The British invented tracer
bullets—bullets which gave off small amounts of flammable material that left a
phosphorescent trail. The first attempt, in 1915, wasn't actually that useful,
as the trail was not straight and limited to 100 meters, but the second tracer
model developed in 1916, emitted a regular bright"
https://infogr.am/harashvivek_1393537750